AbstractComplex Event Processing (CEP) is a technology with support for matching patterns in a cloud or streams of events in order to support detection of specific combinations of event occurrences. A clever specification of event patterns may, for example, detect fraud attempts in a banking system, fire an alarm in response to hazardous situations in a control system or report suspicious customer behavior. Several CEP engines have support for graphically modelling applications as well as perform tests and provide execution traces to verify the application behavior. We argue that it is beneficial to complement testing with formal verification in order to detect errors in early stages of development. In this paper, we present the research prototype tool REX. REX is built as a loosely coupled front end to the timedautomata CASE tool Uppaal. CEP applications and application specific properties can be specified in REX. To support formal verification, REX seamlessly transforms the CEP application together with the specified properties to the timed automata CASE tool Uppaal where the properties are verified by the model-checker provided by Uppaal.